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SEDIMENT REDUCTION THROUGH WATERSHED REHABILITATION by Edward L

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SEDIMENT REDUCTION THROUGH WATERSHED REHABILITATION by Edward L This file was created by scanning the printed publication. Errors identified by the software have been corrected; however, some errors may remain. SEDIMENT REDUCTION THROUGH WATERSHED REHABILITATION By Edward L. Noble U. S. DEPARTMENT OF AGRICULTURE FOREST SERVICE INTERMOUNTAIN REGION SEDIMENT REDUCTION THROUGH WATERSHED REHABILITATION~/ Edward L. NObleg/ History is replete w~th stories recounting the failures of man to recognize, control, and conquer the devastating effects of sediments from steep mountainous lands. Learned men have documented the reasons for the tragic downfall of highly developed civilizations in Mesopotamia, Israel, Egypt, and elsewhere. Most agree that it was not conquest of the land by an invader, nor the loss of fertile fields which depopulated the land, but that the relentless encroachment of silt into the canals and rivers forced the people to move elsewhere or starve. We may now say 1,000 years later that such an occurrence could not happen againj that our engineering skills and general knowledge preclude such disasters that befell the ancient civilizations. In part, such an assumption is true for we have expended billions of dollars to construct dams, levees, canals, and intricate irrigation systems. With such developments, we have assured ourselves of a continuing expansion of irrigated agriculture, industry, and attendant municipalities. There is no question of our awareness of the critical water shortage problem which faces America today. Such awareness was well documented by the voluminous testimony before the Senate Select Committee on National Water Resources. Nor is there a question of the need for continued development of entire river basins to secure water for the grOwing indus­ trial and population expansion which is so imminent. Such recognition is well reflected by the huge annual appropriation of funds for water development and flood control purposes by both Federal and State Governments. With such concrete recognition of the water problems and the vast programs now ~~derway and planned, you may reflect that we have or will have the situation under control and that the continued construction of dams and other structural projects will provide the utopia which we are presumably seeking. T.~ere is no argument that such programs are vital and essenti~l to insure an expanding economy and continuing prosperity. However, there is a question of their continued success if we ignore the need for onsite uses of water and fail to recognize the hydrologic conditions on the watersheds themselves. 1/ Paper prepared for the Federal Interagency Sedimentation Conference, Jackson, MissiSSippi, January 31, 1963. 2/ Forester, Intermountain Region, U.S. Department of Agriculture, Forest SerVice, Ogden, Utah. l The principle of regulatory stream behavior and maintaining soil ; stability through land management is not new. It was advanced as a guiding principle over 50 years ago by the late geologist, Dr. Thomas C. Chamberlin, who stated, "The key lies in due control of the water I-Ihich falls on each acre. • The highest crop value will usually be secured where the soil is made to absorb as much rainfall and snowfall as practical. • . This gives a minimum of wash to foul the streams, to spread over the bottom-lands, to clog the reservoirs, to waste the water power, and to bar up the navigable rivers.!! Since the time of Dr. Chamberlin I s keen observation, we have learned much regarding the hydrology of watershed lands. Research has probed the depths of the soil and critically examined the inter-relationships between soil cover, preCipitation patterns, and water disposal. By so doing, they have established the fact that a very fundamental relation­ ship exists between land condition and hydrologic behavior. We have also learned through experience and the practical application of research results that upstream engineering and improvement of watershed conditions can greatly reduce land erosion and the resultant damages of sedimentation. Recognizing then, the fundamental relationships between land condition and watershed behaVior, we are able to set as our objective the maintenance of both the productive and the hydrologic or water­ regulating functions of the land. By achieving this objective, we are able to obtain the greatest quantity of forage and fiber from the land and simultaneously make certain that water is yielded with the greatest possible regularity and with the least possible load of sediment. In developing a program of sediment reduction through watershed rehabilitation it is necessary to ascertain fundamental facts such as (1) the geologic norm, (2) type of flooding, (3) watershed protection requirements, and (4) adaptability of the site for treatment. GEOLOGIC NORM Essential to the formulation of any flood and erosion control program through watershed rehabilitation is an understanding of the relation/between current flooding and erosion to the geologic norm (1, 3).1 This is necessary as watershed rehabilitation measures are primarily aimed at controlling water runoff and erosion from those areas on which accelerated erosion is occurring. Watersheds with their soil and plant mantle, topography, and streamflow characteristics have been inherited from the geologic past. Their streams exhibit great natural variation in erosion and flood 11 Numbers in parentheses refer to Literature Cited. -2­ ------------------------ behavior. Some streams are usually clear and flow with a relatively constant volume, but the regimen of other streams is marked by great variance in volume and time of flow and vast differences in sediment con­ tent. Each stream is the resultant of such normal factors and forces as climate, topography, geology, and the plant and soil mantle. All these factors and forces have operated through the ages to give use to definite land forms, specific soils, vegetation complexes, and characteristic stream channels, streamflow, and sediment loads. Erosional processes at varying rates on the watershed slopes and in the channels are also part of the norm. We know, for example, that erosion is proceeding so slowly in some areas that soil is being formed and accumulated more rapidly than it is being removed. Streams from such areas carry only negligible loads of sediment. We know, too, that in other areas climatic and geologic conditions limit soil formation, plant growth, and fixing of the land surface. From these drainages runoff has always been rapid and erosion pronounced, giving rise to muddy and highly fluctuating streams. More­ over, we know that between these extremes are all gradations of watershed and sedimentation rates (6). Variation in sediment production in relation to watershed conditions are shown in Figure 1. Figure l.--A. Morris Creek Watershed, Utah, north­ facing basin whose average slope is 48 per­ cent with extreme gradi­ ents of more than 80 percent. Precipitation averages 30 inches an­ nually which is completely infiltrated. Dense vege­ tation provides protection against erosion. Sedi­ ment production from this watershed is only 0.0025 acre-foot per square mile per year and represents a low geologic norm. (6) -3­ Figure l.--B. Lost Creek Watershed, Utahj 85 percent of area com­ prised of steep barren slopes. Active erosion and periodic flooding are characteristic of this basin giving it a high geologic norm. Small islands of soil protected by vegetation show no evidence of overland flow and erosion. Erosion removes soil material as rapidly as it forms from unvegetated slopes. (6) '~~JC";:.';.:..• ~'.' ~:;g From some ~~owledge of the geologic norm, a study of the condition of the watershed, and a determination of the history of runoff from a flooding stream, we can determine whether erosion is accelerated or normal. If accelerated, an opportunity for rehabilitation exists. TYPE OF FLOODING Flooding from high mountain watersheds usually occurs as wet mantle floods due to rapid snowmelt runoff or from dry mantle floods resulting from high intensity summer rainstorms. Although watershed rehabilitation measures have proved valuable in preventing serious damage from both types, they are primarily used to control surface runoff of water and erosion from floods classed as dry mantle. Accordingly, a knowledge of the characteristics of dry and wet mantle floods is necessary in order to design appropriate treatment measures. -4­ Some Characteristics of Dry ~antle and Wet ~antle Floods (7) Factor Dry .Mantle Flood Wet .Mantle Flood Soil mantle condition.............Dry--high water storage capacity.......•.Wet--storage capacity exhausted Precipitation........•..Short intense rainfall...•.•.•.........Prolonged rain­ fall, snowme 1 t or both Storm area......•.•.....Usually small--may be only 5 to 10 percent of flood­ ing drainage ..........•..Relatively large, usually all of flooding drainage Volume of water...••....Relatively small-­ may be only a few acre-feet............Relatively large-­ thousands of acre-feet Manner of flow to stream channels ••••.•.Over surface ...............J:.1a.inly seepage or "bleeding" of saturated soil mantle Sediment carried........High--may reach 60 percent of volume ...•..•.Low--in relation to water volume -5­ I I WATERSHED PROTECTION REQUIREMENTS During the past 40 years, much research work has been done on high mountain watershed lands to determine the relationship between ground­ cover (including plants and litter), precipitation, surface runoff, and erosion. Following floods, field investigations have been made to determine flood-source areas,
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